1. Field of the Invention
The present invention relates to an image forming device such as a laser printer.
2. Description of the Related Art
Laser printers and other image forming devices have been known that use a non-magnetic, single-component toner as an image developer. Such an image forming device includes a photosensitive drum and various components located around the photosensitive drum. The various components include a charge unit, a scanner unit, a developing roller, and a transfer roller disposed in this order around the photosensitive drum following the rotational direction of the photosensitive drum. The photosensitive drum is rotated so that the surface of the photosensitive drum moves to the charge unit, where it is uniformly charged, then to the scanner unit, where it is exposed by a high speed scanned laser beam to form a static-electric latent image on the surface of the photosensitive drum based on image data.
On the other hand, a thin layer of non-magnetic, single-component toner is borne on the developing roller. When rotation of the photosensitive drum brings the toner borne on the developing roller into confrontation with the static-electric latent image formed on the surface of the photosensitive drum, the toner is selectively borne on the static-electric latent image, thereby developing the static-electric latent image into a visible toner image. Then further rotation of the photosensitive drum brings the visible toner image borne on the surface of the photosensitive drum into confrontation with the transfer roller. The visible toner image is transferred to a sheet passing between the photosensitive drum and the transfer roller.
After the visible toner image is transferred to the sheet, paper dust (particularly filler), toner (particularly external additive), and the like that remains on the photosensitive drum can sometimes cling to the surface of the photosensitive drum. This is referred to as xe2x80x9cfilming.xe2x80x9d Filming can reduce the quality of images and also reduce the life of the photosensitive drum.
Impression development devices are particularly susceptible to filming problems. Impression development devices are image forming devices that use non-magnetic, single-component toner and that develop the static-electric latent image by contact between the photosensitive drum and the developing roller. However, because images are developed by scraping the toner borne on the developing roller against the photosensitive drum, abrasion from the toner can greatly damage the surface of the photosensitive drum and filming is particularly likely to occur.
One known method for removing filming from the photosensitive drum is to provide a separate roller made from a resilient member for scraping against the photosensitive drum to remove filming. However, providing a separate roller in this manner increases the size and production expense of the image forming device.
U.S. Pat. No. 5,287,150 describes setting the peripheral speed of a photosensitive drum to 50 mm/sec and the peripheral speed of the developing roller to 70 mm/sec. Because the developing roller rotates faster than the photosensitive drum, the developing roller polishes the filming from the surface of the photosensitive drum.
It is conceivable that the peripheral speed of the developing roller can be changed during periods other than image forming periods, so that during these periods a difference in peripheral speed is generated between the developing roller and the photosensitive drum, and consequently the developing roller polishes the surface of the photosensitive drum. However, in a non-magnetic, single-component developing method, the developing roller is driven to rotate to form an optimum toner layer during image forming periods. If the rotational speed of the developing roller is changed, formation of the toner layer on the developing roller will be unstable, that is, formed with a different thickness than normal For example, if the rotational speed of the developing roller is increased, then the toner layer will be thinner than normal. Similarly, if the rotational speed of the developing roller is decreased, then the toner layer will be thicker than normal. Also, the amount of toner that the developing roller supplies to the photosensitive drum also changes with the peripheral speed of the developing roller. Because the developing mechanism is designed assuming that the developing roller will rotate at a particular speed, if the rotational speed is changed, then the ratio of improperly charged toner will increase.
Although toner is not uniformly charged when it is first supplied to the developing roller, the toner on the developing roller is charge to the same polarity by friction charging by the thickness regulating blade. If the thickness of the toner layer on the developing roller changes, then the ratio of improperly charged toner, such as toner that is charged to the opposite charge than that required for image formation, can increase. This oppositely charged toner can undesirably cling to the photosensitive drum and cause poor quality image formation.
It is an objective of the present invention to overcome the above-described problems and provide a non-magnetic, single-component type image forming device with a simple configuration capable of removing filming from the photosensitive drum so that good-quality images can be formed.
To achieve the above-described objectives, an image forming device according to the present invention includes a photosensitive drum and a developing roller disposed in confrontation and in contact with each other. The photosensitive drum is adapted for supporting a static-electric latent image on its surface and the developing roller is adapted to bear developer. The image forming device further includes a photosensitive drum driver and a developing roller driver for driving rotation of the photosensitive drum and the developing roller, respectively. A drive controller is provided that controls the developing roller driver to one of stop driving the developing roller and maintain the developing roller in a non-rotating condition, while controlling the photosensitive drum driver to drive the photosensitive drum to rotate.
With this configuration, the surface of the rotating photosensitive drum rubs against the developing roller while the developing roller is stopped. This polishes off any filming from the surface of the photosensitive drum, without the need to provide a separate cleaning member for cleaning the surface of the photosensitive drum. As a result, filming can be properly removed and the image forming device can be made more compact and less expensively.
Further, because the developing roller is stopped, and not merely sped up or slowed down, no unstable toner layer will be formed on the developing roller while the developing roller is stopped. An optimum toner layer can always be formed on the developing roller, so that good images can be formed using a non-magnetic, single component developing method.
A method according to the present invention is for removing film from the surface of a photosensitive drum that is in contact with a developing roller. The method includes starting rotation of the photosensitive drum while the developing roller is maintained in an unmoving condition so that surface of the photosensitive drum rubs against the developing roller; and subsequently starting rotation of the developing roller.
A method according to another aspect of the present invention includes starting rotation of the photosensitive drum and the developing roller substantially simultaneously; and subsequently stopping rotation of the developing roller to bring the developing roller into an unmoving condition while maintaining the photosensitive drum in a rotating condition so that surface of the photosensitive drum rubs against the developing roller.